Mobility control in secondary-type oil recovery is dependent upon many factors which have different degrees of effect on the reservoir. Some of the most significant factors are the formation rock type, the permeability range, the mobility control agent, solution viscosity and the connate water chemistry. If the formation rock type and connate water composition are known, and if the polymer resistance factor can be adjusted without unduly plugging the formation, mobility control can be optimized.
A low concentration of high molecular weight, water-soluble polymer improves the mobility control of water injected into oil reservoirs. Reduced water mobility occurs partially by viscosity increase and partially by reduction of the reservoir permeability. Reduction of permeability is of different magnitudes for various polymer solutions. Retention by mechanical entrapment occurs at pore constrictions and accounts for most of the permeability reduction but adsorption also causes some of the polymer to be retained. The concentration difference between the entering and leaving polymer solutions in the reservoir approaches zero as stabilization occurs. Finally stabilization takes place and the permeability remains constant at a lower than original level. After stabilization, the polymer solution injected into the reservoir moves through the large pore openings. If the polymer molecules are either too large, extensively cross-linked or extensively entangled, retention of polymer molecules can cause more plugging than desired.
Polymers available on the market will seldom provide the exact solution properties optimum for a given flooding operation. Degrading substantially all the molecules of a polymer available on the market with an exessively high average molecular weight is a viable method of obtaining the qualities necessary for a flooding operation. There are many methods taught in the prior art which relate to or attempt to alleviate the problem.
U.S. Pat. No. 3,709,297 teaches the injection of a slug of dilute aqueous polyacrylamide solution into an oil-containing formation followed by injection of a slug of dilute aqueous solution of an alkali metal hypochlorite to improve the injectivity.
U.S. Pat. No. 3,343,601 teaches the addition of water-soluble hydrosulfite, an oxygen scavenger, to a polymer mobility control agent to reduce the plugging of formations by ferric hydroxide.
U.S. Pat. No. 3,842,909 issued to Rhudy et al teaches a process wherein a portion of a polymer having an average molecular weight at least 1.2 times greater than the actual molecular weight needed to make up an aqueous solution is degraded mechanically prior to injection.
U.S. Pat. No. 3,556,221 teaches the use of NaOCl to reduce plugging adjacent a well bore.
The presence of free oxygen or oxidizing agents in the polymer solution is identified with polymer degradation. Reducing agents are used to counteract the effect of polymer degradation induced by oxidation-reduction reactions. None of the art, however, combines these agents to ensure that the resultant solution provides effective mobility control. The limitations of the prior art are overcome by the present invention which insures optimum mobility control without excessive plugging of the reservoir.